Beneficiation of phosphate rock



3,086,654 Patented Apr. 23, 1963 3,086,654 BENEFICIATION F PHOSPHATE ROCK Clinton A. Hollingsworth, Lakeland, and Bobby L. Sapp, Plant City, Fia., assignors to Smith-Douglass Company, Incorporated, Norfolk, Va., a corporation of Virginia Filed Dec. 15, 1960, Ser. No. 76,065 6 Claims. (Cl. 209166) This invention relates to the beneficiation of phosphate rock and similar phosphatic materials, and in particular to an improved method of flotation of the phosphate rock to obtain a high grade phosphate concentrate.

Phosphate rock and other similar phosphatic minerals commonly are found in large natural deposits admixed with such gangue materials as sand, organic matter and the like. The phosphatic constituents of these naturally occurring materials is a valuable article of commerce, and a great deal of effort has been devoted to the problem of beneficiating or concentrating these phosphatic constituents by removal of the gangue material therefrom.

In the usual process for beneficiating such phosphatecontaining material (hereinafter referred to collectively as phosphate rock), the mixture of phosphatic minerals and gangue obtained from the phosphate deposit or mine is first washed to remove trash therefrom and to separate and recover the larger lumps of phosphate minerals (usually referred to as pebble rock). The remaining material (hereinafter referred to as the washer reject material) consists of a mixture of phosphate rock particles, sand and similar gangue materials substantially all of which is less than about 10 mesh (Tyler Standard), and preferably is less than 14 mesh, in size. The phosphate rock constituent of the washer reject material is then concentrated and recovered for subsequent use, commonly by a combination of froth and table flotation techniques.

The need for employing a combination of froth and table flotation techniques to effect concentration of the phosphate constituents of the washer reject material stems from the fact that it has not been feasible heretofore to float by froth flotation any significant amount of the coarse (plus 35 mesh) washer reject material, and particularly coarse silica having a particle size of more than about 35 mesh. Consequently, it has heretofore been the practice to size the washer rejects to obtain a coarse fraction having a particle size of between about minus 14 mesh and plus 35 mesh and a fine fraction having a particle size between about minus 35 mesh and plus 150 mesh. The phosphate values of the coarse fraction, not heretofore susceptible to concentration by froth flotation, are now commonly concentrated and recovered by means of table or belt flotation techniques. On the other hand, the' phosphate values of the fine fraction of the washer rejects are susceptible to concentration and recovery by froth flotation techniques well known in the art, and these techniques are commonly employed for this purpose. As a result, the process currently employed in commercial practice for the beneficiation of the washer reject material requires the installation and maintenance of an elaborate and expensive flotation plant equipped to carry out both table and froth flotation operations. Moreover, the amount of phosphate values recovered by the combination of flotation operations is, at best, limited to about 83 to 88% by weight of the phosphate content of the initial deslimed washer reject material.

Specifically, the most successful process heretofore employed for the froth flotation of phosphate rock, and the one currently almost universally employed by the phosphate industry for this purpose, comprises forming an aqueous pulp of the sized, deslimed phosphate-containing material, and subjecting the aqueous pulp to flotation with an anionic flotation reagent to collect and float from the aqueous pulp a rougher concentrate containing a high proportion of phosphate values mixed with some of the siliceous gangue. The rougher phosphate concentrate is then deoiled to remove the anionic reagent, is again deslimed to remove the accumulation of very fine particles or slimes unavoidably produced during the first flotation step and subsequent deoiling operation, and is then subjected to flotation with a cationic flotation reagent (such as one of the known amine reagents) to collect and float therefrom most of the aforementioned siliceous gangue contained in the rougher concentrate which is discarded as tails. The underflow from the second flotation step comprises a high grade, commercially valuable phosphate concentrate. Alternatively, it has also heretofore been proposed that the aforementioned sequence of anionic and cationic flotation steps be reversed, but the inherent limitations and inefliciencies of the froth flotation operation remain essentially unchanged. That is to say, all prior froth flotation techniques have been limited in their application to the concentration of the phosphate rock constituent of the washer reject material having a particle size of less than about 35 mesh, and the overall phosphate recovery is only about 8388% by weight of the phosphate content of the initial deslimed washer reject material.

It has long been recognized that it would be more ef-.

ficient and economical to effect concentration and recovery of the phosphate rock constituent of the washer reject material by subjecting all of this material to a single type of flotation operation, preferably froth flotation, rather than to a combination of such operations, such as the aforementioned combination froth and table flotation.

As a result of an intensive laboratory and pilot plant investigation of various flotation techniques we discovered a new process that is described and claimed in our copending application Serial No. 832,077, filed August 6, 1959, now Patent No. 3,013,664, dated December 19, 1961, by means of which the phosphate constituent of the washer reject material can be efficiently and economically recovered by froth flotation techniques without the necessity for sizing the washer rejects to split it into a fine fraction and a coarse fraction suitable for concentration respectively by the aforementioned known combination of froth flotation techniques and table or belt flotation techniques. This process is based on our important discovery that the coarse particles of the washer reject material (i.e. particles between about minus 14 mesh and plus 35 mesh), and particularly the coarse silica particles, can be treated in such a way as to become activated so that these particles can be floated by con ventional flotation reagents. Specifically, we discovered that the coarse particles of the Washer rejects become activated when these particles are recirculated through a two-stage flotation operation comprising a bulk silica float followed by a phosphate float. The continuous recirculation of the underflow or tails from the second or phosphate flotation step not only results in the flotation of activated coarse silica particles but, in addition, permits recovery of the coarse and other diflicult to float phosphatic material, and contrary to normal expectations there is no appreciable build-up of silica or other gangue marterials or of difficult to float phosphatic minerals due to the continuous recirculation of the underflow from the second flotation step of this process.

We have now found that an important improvement in the aforementioned process is obtained when the underfiow or tails from the second or phosphate flotation step is not recirculated to the first or silica flotation step but instead is subjected to a third and separate silica flotation operation, the underflow from this third flotation step then being recirculated to the second or phosphate flotatiou step of the process. The bulk of the fine, easy to float silica is removed from the feed material during the first silica flotation step (referred to herein as the primary silicia float circuit) and the bulk of the fine, easy to float phosphate material is removed during the second or phosphate flotation step of the process, the silica and phosphate remaining in the underflow from the second flotation step being predominantly coarse, normally unfloatable particles of silica and phosphate. By subjecting the underflow from the second or phosphate flotation step to a separate silica flotation step (referred to herein as the secondary silica float circuit) several important advantages are obtained. First, as the bulk of the fine phosphate has been removed during the second flotation step, there is less danger of phosphate entrainment in the silica floated during the third flotation step. Furthermore, since the raw feed usually contains more phosphate and slimes than the underflow from the second or phosphate flotation step, and since silica flotation reagent consumption (i.e., cationic reagent consumption) is related more to phosphate and slime content than it is to silica content, the silica contained in the underflow from the second flotation step can be floated more economically by our new process than is possible when it is recirculated and mixed with the raw feed entering the first flotation step as is the case in our prior process. A further important advantage is that, if desired, the flotation reagent used in the secondary silica circuit (i.e. the third flotation step) may be different from the reagent used in the primary silica circuit (i.e. the first flotation step) which, in many cases, can result in a significant reduction in reagent cost.

Accordingly, our improved process for beneficiating phosphate rock having a particle size of between about minus 14 mesh and plus 150 mesh (i.e. the deslimed washer reject material) comprises subjecting an aqueous pulp of the rock to a first flotation operation with a cationic flotation reagent to float from the aqueous pulp substantially all of the fine silica present in the pulp, the silica floated being discarded as tailings. The underflow from the bulk silica float, referred to herein as the rougher phosphate concentrate, contains substantially all of the phosphate values of the rock, unfloated coarse silica and other gangue material. The rougher phosphate concentrate is conditioned with an anionic flotation reagent and is then subjected to a second froth flotation operation to collect and float a phosphate concentrate which is recovered as the desired product of the process. The underflow from the second flotation operation which contains coarse silica and difficult to float phosphate particles is subjected to a third separate and distinct froth flotation operation with a cationic flotation reagent to float particles of activated coarse silica, the silica floated during this flotation operation being discarded as tails. The underflow from the third flotation operation is recycled to the anionic conditioning operation and thereafter successively through the second and third flotation steps of the process whereby difficult to float phosphate particles and activated coarse silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process. As a result of our improved process, phosphate recoveries in excess of about 95% by weight of the initial phosphate content of the pulp are commonly obtained at a reagent cost of about one-half to two-thirds of that required by our prior two-step flotation process.

Our new process will be more fully described in connection with the accompanying schematic drawing of a plant equipped to carry out our process on a commercial scale.

The raw feed, which comprises an aqueous pulp of plant heads or washer reject material having a particle size of less than about 14 mesh (Tyler Standard Screen), is first deslimed by passing it through a desliming cyclone 3, is again screened to scalp out trash and over-sized particles, and is again deslimed in a hydro-deslimer 5 to remove any slimes which pass through the desliming cyclone or which were produced as a result of the screening operation. The screened and deslimed raw feed material having a particle size of between about -14 and mesh is then introduced into the first of a series of bulk silica flotation cells 6 together with suflicient water to form an aqueous pulp of suitable density for flotation and a suflicient amount of a cationic flotation reagent (e.g. a conventional amine-type reagent) to effect flotation of the fine silica (between about minus 35 and plus 150 mesh) content of the pulp.

The presence of slimes in the aqueous pulp during the bulk silica float has a serious adverse effect on both the selectivity and the efficiency of conventional cationic reagents as collectors of siliceous material. Accordingly, the first or bulk silica flotation step of our new process is advantageously carried out in froth flotation cells that minimize the formation of slimes during the flotation operation, and We have found that the type of hydraulicpneumatic flotation cell described in U.S. Patent 2,758,- 714 to Clinton A. Hollingsworth is most satisfactory for this purpose. As will be seen from the schematic drawing, a series of three of such hydraulic-pneumatic cells 6 (namely cells 6a, 6b and 6c) are advantageously employed to carry out the bulk silica float. As previously mentioned, the mixture of raw feed and recirculated material is introduced into the first cell 6a of the series of cells, the overflow or float from this cell comprising predominantly fine silica and the underflow or first rough er concentrate comprising the phosphatic material, unactivated coarse silica and other unfloated material. The underflow from the first cell is introduced into the second or rougher concentrate cleaner cell 6b of the series of cells, the overflow or float from this cell comprising fine silica that was not floated in the first cell 6a and the underflow comprising the final rougher phosphate concentrate that is to be subjected to anionic flotation in the second flotation step of our process. The overflow from the first and second cells is introduced into the third or tailing cleaner cell 60 of the series of cells, the overflow from this cell being discarded as tailings and the underflow being delivered to the middling tank or sump 7 for recycling through the first flotation step of our process.

The rougher phosphate concentrate from the second silica float cell 6b comprises essentially the unfloated phosphatic material and unactivated coarse silica. This concentrate is delivered to and stored in a surge tank or sump 9 prior to being dewatered, advantageously in a dewatering cyclone 11, and delivered to a conditioner 12. The rougher concentrate is conditioned for the second step of our flotation operation by admixing it with one of the well-known anionic flotation reagents (e.g., a combination of fatty acids, caustic soda and fuel oil) used for this purpose in the phosphate industry. The conditioner rougher concentrate is then introduced into the first of a series of phosphate flotation cells 14.

The flotation of phosphate minerals is not greatly affected by the presence of slimes in the phosphate flotation cell. As a consequence, any of the froth flotation cells heretofore employed in the phosphate industry for the flotation of phosphate rock can be employed in the second step of our new process. However, we presently prefer to employ the type of hydraulic-pneumatic cell described in Patent 2,753,045 to Clinton A. Hollingsworth.

The second step of our flotation operation is advantageously carried out in a series of at least two of the aforementioned hydraulic-pneumatic flotation cells 14 (namely, cells 14a and 14b). The conditioner rougher phosphate concentrate is introduced into the first cell 14a of the series of cells, the overflow or float from this cell comprising the phosphate concentrate and the underflow or tails from this cell comprising unactivated coarse silica and difficult to float phosphate particles. The overflow from the first of the series of phosphate float cells is introduced into the second or phosphate concentrate cleaner cell 14b, the overflow or float from this cell be ing the final phosphate concentrate and the underflow or tails from this cell comprising the minor amount of unfloated phosphates, silica or other gangue material, if any, which may have carried over from the first of the phosphate flotation cells, said underflow being referred to in the drawing as Phosphate Midds.

The underflow from the first phosphate flotation cell 140: contains diflicult to float phosphate and unactivated coarse silica together with other unfloated materials. This material is then subjected to a third flotation op eration in which a cationic flotation reagent is employed to float and remove therefrom the activated coarse silica particles. Prior to this secondary silica flotation step, however, these phosphate tails are advantageously dewatered and deslimecl in a cyclone deslimer 17, screened to remove oversized particles, and again deslimed in a hydro-deslimer 19 before being delivered to the silica flotation cell 20. Because of the adverse effect that the presence of slimes have on the efliciency and selectivity of most known cationic flotation reagents, the secondary silica flotation step is advantageously carried out in a cell 20 that minimizes the formation of siimes such, for example, as the hydraulic pneumatic cell described in the aforementioned US. Patent 2,758,714. Moreover, since substantially all of the fine, easy to float particles have been removed from the feed material prior to the third flotation step of our process, the optimum quantity of cationic flotation reagent that must be employed in order to float the activated coarse silica content of the phosphate tails is less when the tails are subjected to a separate secondary silica flotation operation than when the phosphate tails are added to the raw feed materials that are to be subjected to cationic flotation in the primary silica flotation step of our process. In addition, in many cases markedly improved results can be obtained if a different cationic flotation reagent is employed in the secondary silica flotation circuit due to the preferential ability of certain of these reagents to float silica under various conditions. The overflow from the secondary silica flotation circuit comprises predominantly activated coarse silica particles which are discarded as tails, the under flow comprising predominantly unactivated coarse silica particles and difficult to float phosphate particles which are delivered to the tank or sump 9 for recycling through the second and third flotation steps of our process.

By subjecting the underflow or tails from the second or phosphate flotation step of our process to a third or secondary silica flotation step, and by recirculating the underflow from the third flotation step continuously through the second and third steps of our process, we achieve an important and entirely different result than that which ordinarily is to be expected by the mere recirculation of flotation railings. That is to say, the secondary silica flotation step would ordinarily be expected to result in the mere scavenging of normally floatable fine silica particles that somehow escaped flotation during the primary silica flotation step, and the recycling of the tails from the secondary silica flotation step would ordinarily be expected to result in the mere scavenging of normally floatable phosphate particles which somehow escaped flotation in the course of the preceding phosphate flotation step. As a consequence, it would ordinarily be expected that the coarse silica and other diflicult to float particles would build up in the system with the result that eventually the phosphate and secondary silica tailings would have to be discarded without recycling. However, we have found that the recycled coarse silica and other diflicult to float particles do not build up in the system, and we have discovered the reason for this to be that these particles somehow become activated as a result of being repeatedly subjected alternately to cationic and anionic flotation, the activated particles being floated by the corresponding flotation reagent and thereby being removed from the system.

The activation of the coarse silica and other difficult to float particles is a surface phenomenon the mechanism of which is not fully understood. However, the activation of these particles is nonetheless a fact as is evidenced by the appearance of coarse silica in the overflow from the secondary silica flotation cell 20 and by the appearance of difficult to float phosphate particles in the overflow from the phosphate flotation cells 14 after the tailings from the second step of our process have been recycled one or more times through the third and second steps thereof. The surprising activation and flotation of the coarse silica and difficult to float phosphate particles makes it possible to obtain substantially greater recoveries than those achieved by the most efficient flotation processes heretofore employed in the phosphate industry. Moreover, as a result of our improved flotation process the cost of the flotation reagents employed to obtain this exceptionally high phosphate recovery is about one-half to two-thirds of that required by the two-step flotation process described in our copending application Ser. No. 832,077.

The following examples are illustrative but not limitative of the practice of our invention:

A series of comparative tests described in Examples I, II and III were carried out to determine the relative etficiency and economy of the two-stage flotation process described in our copending application Serial No. 832,077 and the improved three-step flotation process described herein.

EXAMPLE I In the first of the series of tests the flotation procedure employed was essentially the same as the two-step process described in our copending application Serial No. 832,077. The raw feed comprised washed, screened and de-slimed plant heads or washer reject material having a particle size of less than about 14 mesh (Tyler Standard) and containing an average of 25.46% by weight of tricalcium phosphate (referred to as the bone phosphate of lime or B.P.L. content of the material). The feed material (together with recirculated silica midds and phosphate tails) was subjected to froth flotation with a conventional amine type cationic flotation reagent to float the silica content thereof. The overflow or silica tails from the cationic flotation operation contained fine silica and activated coarse silica which, after being cleaned by a supplementary cationic flotation operation, were discarded as waste, the discarded silica tails containing 1.19% by weight of B.P.L. The underflow or rougher phosphate concentrate from the cationic flotation operation was conditioned with an anionic flotation reagent, and the conditioned rougher concentrate was then subjected to froth flotation to float the phosphate content of the feed material. The overflow from the anionic flotation operation contained easy to float fine phosphate and activated difficult to float coarse phosphate particles which, after being cleaned by a supplementary anionic flotation operation, were recovered as the desired phosphate concentrate, the phosphate concentrate containing 75.42% by weight of B.P.L. and 2.86% by weight of insoluble matter. The underflow or phosphate tails from the anionic flotation operation contained unactivated coarse silica and difficult to float phosphate particles which, after being screened to remove any unfloatable (+14 mesh) phosphate and silica particles, were recycled to the start of the process where they were mixed with the raw feed material entering the initial cationic flotation operation. In addition, the underflow or silica midds from the silica cleaner flotation cell were also recirculated to the start of the process where they were mixed with the raw feed material, and the underflow or phosphate midds from the phosphate cleaner flotation cell were recirculated through the phosphate flotation circuit.

The amount of phosphate in the phosphate concentrate produced by our two-step process was 96.8% of the phosphate content of the initial feed material. The flotation reagent consumption amounted to 4.15 lbs. of cationic reagent per ton of concentrate and 23.2 lbs. of anionic reagent per ton of concentrate which, at current market prices for these reagents, represented a reagent cost of 91.5 cents per ton of concentrate.

EXAMPLE II In the second of the series of tests the flotation procedure employed was essentially the same as that described in Example I with the important exception that the phosphate tails (i.e. the underflow from the anionic flotation operation) were subjected to a separate secondary cationic flotation step instead of being recycled to the start of the flotation process. Thus, the raw feed comprised washed, screened and deslimed plant solids or washed reject material having a particle size of less than about 14 mesh and containing an average of 25.46% by 95.4% of the phosphate content of the initial feed material. The flotation reagent consumption amounted to 2.06 lbs. of cationic reagent per ton of concentrate and 18.9 lbs. of anionic reagent per ton of concentrate which, at current market prices for these reagents, represented a reagent cost of 56.8 cents per ton of concentrate.

EXAMPLE III In the third of the series of tests, the flotation procedure was essentially the same as that described in Example II with the exception that a different cationic flotation reagent was employed in the first or primary silica flotation step than in the third or secondary silica flotation step. Thus, in the first or primary silica flotation step the cationic flotation reagent employed was of the type described in US. Patent 2,857,331 whereas in the third or secondary silica flotation step the cationic flotation reagent was a conventional commercially available cationic reagent. The amount of phosphate in the phosphate concentration produced by this modification of our threestep flotation process was 94.0% of the phosphate content of the initial feed material. The flotation reagent consumption amounted to 1.09 lbs. of cationic reagent per ton of concentrate and 17.9 pounds of anionic reagent per ton of concentrate which, at current market prices for these reagents, represented a reagent cost of 52.8 cents per ton of concentrates produced.

The results of the comparative tests are shown in Table I. The savings in reagent cost in Examples 2 and 3 as compared to Example 1 is readily apparent.

Table I Cone. Corn- Lbs. Reagent/TE. Reagent Feed bined Example Pcrccnt Silica Ratio Percent No. B.P.L. Percent. Percent Tails, cc. Costl B.P.L. Insul. Percent Cationic Anionic T.C.

B.P.L.

25.46 75.42 2.86 1.19 3.03 96.8 4.15 23.2 91.56 25.46 75.06 3.34 1.70 3.00 95.4 2.06 18.9 seas 25.46 75.40 2.89 2.27 3.15 94.0 1.09 17.9 62.8;5

as was employed in Example I, and the conditioned rougher concentrate was then subjected to a second froth flotation step to float the phosphate content of the feed material. The overflow from the anionic flotation operation contained easy to float fine phosphate and activated diflicult to float or coarse phosphate particles which, after being cleaned by a supplementary anionic flotation operation, was recovered as the desired phosphate concentrate, the finished phosphate concentrate containing 75.06% by weight of B.P.L. and 3.34% by weight of insoluble matter. The underflow or phosphate tails from the anionic flotation operation contained unactivated coarse silica and diflicult to float phosphate particles which, after being screened to remove any unfloatable (+14 mesh) phosphate and silica particles, were subjected to a third froth flotation step with the same cationic flotation reagent as was employed in the first flotation step. The overflow or silica tails from this cationic flotation operation contained predominantly activated coarse silica particles which were discarded as waste, the combined silica tails from the first and the third flotation steps containing 1.70% by weight of B.P.L. The underflow from the third or secondary silica flotation step and the underflow from the second or phosphate flotation step were recirculated to the anionic conditioning operation where they were combined with the rougher phosphate concentrate from the first or primary silica flotation step for recycling through the second and third flotation steps of the process. In addition, the underflow or silica midds from the silica cleaner flotation cell were recirculated to the start of the process where they were mixed with the raw feed material. The amount of phosphate in the phosphate concentrate produced by our three-step flotation process was From the foregoing description of our improved process for beneficiating phosphate rock, it is apparent that we have made an important contribution to the art to which our invention relates.

This application is a continuation-in-part of our copending application Serial No. 832,077, now Patent No. 3,013,664, dated December 19, 1961.

We claim:

1. The method of beneficiating phosphate rock having a particle size of from about 14 mesh to about 150 mesh Which comprises subjecting an aqueous pulp of the rock to a first froth flotation operation with cationic flotation reagent to float from the aqueous pulp substantially all of the fine silica, conditioning the rougher phosphate concentrate underflow from the first flotation operation with an anionic flotation reagent, subjecting the rougher concentrate to a second froth flotation operation with said anionic flotation reagent to float a phosphate concentrate which is recovered as the desired phosphate product, the unfloated tailings from the second flotation operation containing coarse silica and diflicult to float phosphate particles, subjecting said unfloated phosphate tailings to a third separate and distinct froth flotation operation with a cationic flotation reagent to float particles of activated coarse silica, and recirculating the unfloated tailings from the third flotation operation to the anionic conditioning operation and successively through the second and third flotation operations of the process whereby (liflicult to float phosphate particles and activated coarse silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process.

2. The method of beneficiating phosphate rock having a particle size of from about 14 mesh to about 150 mesh which comprises subjecting an aqueous pulp of the rock to a first froth flotation operation with a cationic flotation reagent to float from the aqueous pulp substantially all of the silica having a particle size of less than about mesh, conditioning the rougher phosphate concentrate underflow from the first flotation operation with an anionic flotation reagent, subjecting the rougher concentrate to a second froth flotation operation with said anionic flotation reagent to float a phosphate concentrate which is recovered as the desired phosphate product, the unfloated tailings from the second flotation operation containing silica having a particle size of between about 14 mesh and +35 mesh and difficult to float phosphate particles, subjecting said unfloated tailings to a third separate and distinct froth flotation operation with a cationic flotation reagent to float particles of activated silica, and recirculating the unfloated tailings from the third flotation operation to the anionic conditioning operation and successively through the second and third flotation operations of the process whereby difficult to float phosphate particles and activated silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process.

3. The method of beneficiating phosphate rock having a particle size of from about 14 mesh to about 150 mesh which comprises subjecting an aqueous pulp of the rock to a first froth flotation operation with a cationic flotation reagent to float from the aqeuous pulp substantially all of the fine silica, said cationic flotation operation being carried out in a flotation cell that minimizes the formation of slimes, conditioning the rougher phosphate concentrate under-flow from the first flotation operation with an anionic flotation reagent, subjecting the rougher concentrate to a second froth flotation operation with said anionic flotation reagent to float a phosphate concentrate which is recovered as the desired phosphate product, the unfloated tailings from the second flotation operation containing coarse silica and difficult to float phosphate particles, subjecting said unfloated tailings to a third separate and distinct froth flotation operation with a cationic flotation reagent to float particles of activated coarse silica, said cationic flotation operation being carried out in a flotation cell that minimizes the formation of slimes, and recirculating the unfloated tailings from the third flotation operation to the anionic conditioning operation and successively through the second and third flotation operations of the process whereby difficult to float phosphate particles and activated coarse silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process.

4. The method of beneficiating phosphate rock having a particle size of from about 14 mesh to about 150 mesh which comprises subjecting an aqeuous pulp of the rock to a first froth flotation operation with a first cationic flotation reagent to float from the aqueous pulp substantially all of the fine silica, conditioning the rougher phosphate concentrate underflow from the first flotation operation with an anionic flotation reagent, subjecting the rougher concentrate to a second froth flotation operation with said anionic flotation reagent to float a phosphate concentrate which is recovered as the desired phosphate product, the unfioated tailings from the second flotation operation containing coarse silica and diflicult to float phosphate particles, subjecting said unfloated tailings to a third separate and distinct froth flotation operation with a second cationic flotation reagent to float particles of activated coarse silica, and recirculating the unfloated tailings from the third flotation operation to the anionic conditioning operation to the anionic conditioning operation and successively through the second and third flotation operations of the process whereby difiicult to float phosphate particles and activated coarse silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process.

5. The method of beneficiating phosphate rock having a particle size of from about 14 mesh to about mesh which comprises subjecting an aqueous pulp of the rock to a first froth flotation operation with a first cationic flotation reagent to float from the aqueous pulp substantially all of the silica having a particle size of less than about 35 mesh, said cationic flotation operation being carried out in a flotation cell that minimizes the formation of slimes, conditioning the rougher phosphate concentrate underflow from the first flotation operation with an anionic flotation reagent, subjecting the rougher concentrate to a second froth flotation operation with said anionic flotation reagent to float a phosphate concentrate which is recovered as the desired phosphate product, the unfloated tailings from the second flotation operation conraining silica having a particle size of between about 14 mesh and +35 mesh and diflicult to float phosphate particles, subjecting said unfloated tailings to a third separate and distinct froth flotation operation with a second cationic flotation reagent to float particles of activated silica, said secondary cationic flotation operation being carried out in a flotation cell that minimizes the formation of slimes, and recirculating the unfloated tailings from the third flotation operation to the anionic conditioning operation and successively through the second and third flotation operations of the process whereby diflicult to float phosphate particles and activated silica particles are floated during the succeeding phosphate flotation and silica flotation steps of the process.

6. The method according to claim 5 in which the cationic flotation operations are carried out in hydraulicpneumatic flotation cells.

References Cited in the file of this patent UNITED STATES PATENTS 2,914,173 Le Baron Nov. 24, 1959 2,927,692 Hollingsworth Mar. 8, 1960 2,970,688 Uhland Feb. 7, 1961 3,013,664 Hollingsworth Dec. 19, 1961 

1. THE METHOD OF BENEFICIATING PHOSPHATE ROCK HAVING A PARTICLE SIZE OF FROM ABOUT 14 MESH TO ABOUT 150 MESH WHICH COMPRISES SUBJECTING AND AQUEOUS PULP OF THE ROCK TO A FIRST FROTH FLOTATION OPERATION WITH CATIONIC FLOTATION REAGENT TO FLOAT FROM THE AQUEOUS PULP SUBSTANTIALLY ALL OF THE FINE SILICA, CONDITIONING THE ROUGHER PHOSPHATE CONCENTRATE UNDERFLOW FROM THE FIRST FLOTATION OPERATION WITH AN ANIONIC FLOTATION REAGENT, SUBJECTING THE ROUGHER CONCENTRATE TO A SECOND FROTH FLOTATION OPERATION WITH SAID ANIONIC FLOTATION REAGENT TO FLOAT A PHOSPHATE CONCENTRATE WHICH IS RECOVERED AS THE DESIRED PHOSPHATE PRODUCT, THE UNFLOTATED TAILINGS FROM THE SECOND FLOTATION OPERATION CONTAINING COARSE SILICA AND DIFFICULT TO FLOAT PHOSPHATE PARTICLES, SUBJECTING SAID UNFLOATED PHOSPHATE TAILINGS TO A THIRD SEPARATE AND DISTINCT FROTH FLOTATION OPERATION WITH A CATIONIC FLOTATION REAGENT TO FLOAT PARTICLES OF ACTIVATED COARSE SILICA, AND RECIRCULATING THE UNFLOATED TAILINGS FROM 